Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are associated with adaptive function impairments. This study measured adaptive functioning in a sample of 92 children with ASD, including 80 children with co-occurring ADHD. Regression models examined whether ASD and ADHD symptom severity were associated with adaptive functioning level relative to same-age peers (standard scores). ASD severity predicted weaker overall adaptive functioning, relative to peers; ASD and ADHD severity each independently predicted weaker adaptive function in Socialization. Thus, severity of ADHD in children with ASD is associated with greater adaptive deficits than what would be expected on the basis of ASD severity alone. In contrast, ADHD was not significantly related to greater deficits in other adaptive domains, above and beyond that associated with ASD. Clinicians should be aware that ADHD confers additional developmental risk in the area of socialization in children with ASD, and that treatment plans should be designed accordingly.